The present invention relates to tube expansion, and more particularly to controlled expansion of a tube within a tube sheet or within another tube using a pressurized fluid system.
Steam generators used in commercial nuclear power plants are heat exchangers including a vessel containing a large number of stainless steel tubes affixed at their ends to tube sheets. In some steam generators, namely, the "U-tube" type, the tubes are formed in the shape of a "U" with both ends affixed to a single tube sheet. In other steam generators, namely, the "once-through" type, the tubes are straight and affixed between two separate tube sheets. Typically, heated radioactive high pressure reactor core primary coolant is directed through the tubes. A relatively cool, low pressure secondary coolant, typically water, is pumped through the steam generator around the hot tubes to thereby gain heat and to vaporize into steam, thus the name "steam generator." The steam generator tubes are exposed to a hostile atmosphere of undesirable chemicals, temperatures and temperature gradients that result in the degradation of the tubes' integrity. For example, corrosive chemical action that occurs during alternate wetting and drying of the tube surface in a vapor-liquid mixture atmosphere leads to a failure mechanism known as stress-corrosion cracking. Another mechanism leading to tube failure is vibration induced errosion.
However, regardless of how the steam generator tube fails, the result is a leak and a flow of radioactive high pressure primary coolant into the low pressure secondary coolant. A certain number of these leaks are tolerable. However, when leakage occurs to the extent that the secondary coolant becomes unacceptably radioactive it becomes necessary to replace or plug the tubes. In that replacing tubes is a difficult operation, especially in the case of the U-tube type steam generator, the tubes are typically plugged. Unfortunately, as more tubes are plugged, the capacity of the steam generator is decreased. Eventually, the capacity of the steam generator will be decreased to such a degree that large scale overhaul is required.
The foregoing inevitability can be circumvented to some extent by stiffening the tubes in the vicinities of defects before the defects become leaks. Hereinafter, a "defective tube" is defined as a tube having a degraded wall thickness but not a leaking tube. It may be desirable to plug the tube rather than stiffening it once the defect has surpassed 40% of the wall thickness. Defective tubes can be identified by known tube inspection techniques. Furthermore, as a precautionary measure it is desirable to stiffen tubes in areas of the steam generator which experience high fluid velocities where the likelihood of vibration induced erosion is increased.
To stiffen the tubes, typically, a sleeve of sufficient length to cover the defect and to allow expansion of the sleeve into the tube above and below the defect is inserted within the tube and positioned at the defect location. The sleeve and tube are then expanded above and below the defect to hold them together and thereby stiffen the defective portion of the steam generator tube.
Several methods and devices are available in the prior art for expanding the sleeve within the tube. Rogers, Jr. et al (U.S. Pat. No. 4,069,573) described a hydraulic tube expander that applies fluid pressure to the inside of the sleeve to expand it into the steam generator tube. Hereinafter, the term "hydraulic" expander refers to an expander utilizing fluid (liquid or gas) pressure to effect expansion. In Rogers, Jr. et al, a set expansion pressure is first applied within the fixed tube, then an additional fixed volume of fluid is forced into the system volume. This method and device suffer from several drawbacks. First, the fluid is applied directly to the inside of the sleeve. This requires a good seal between the expander device and the sleeve, thus, an accurate sizing of the sleeve's inside diameter is critical. Also, the sleeve's inside surface must be extremely smooth. These requirements add significantly to the sleeve cost. Second, this device spills undesirable fluid into the steam generator necessitating clean-up and repriming of the apparatus before the next expansion. Third, the method of applying a fixed fluid pressure followed by a fixed volume input results in a steam generator tube outside diameter increase which is controllable to within about 0.025 inches. This degree of expansion control is not acceptable if the tubes are ever to be withdrawn from the tube sheets for replacement i.e., when enough tubes are damaged to so warrant rebuilding of the steam generator. An expansion of 0.025 inches will preclude withdrawal of the tube without an unacceptable risk of damage to the tube sheet. This is a particular problem in the once through steam generator wherein the only way to remove the tube is through a tube sheet. An acceptable degree of steam generator tube outer diameter expansion control is about 0.006 inches or less, which will allow withdrawal of the steam generator tubes through the tube sheet.
The reason the method of the prior art cannot achieve the desired expansion control is that, because of variance in the dimension and yield strengths of the sleeves and the tubes, one cannot calculate what fluid pressure to apply to the system or volume of fluid to inject into the system, unless the dimensions and yield strengths of the particular sleeve and tube undergoing expansion are known. Unfortunately, these values vary due to manufacturing tolerances and in-service material property transitions. Each case is different. Treating each expansion uniformly as in the prior art limits control of the steam generator tube outer diameter to about within 0.025 inches. Therefore, one cannot calculate the fluid pressure, or volume of fluid introduced to the system or decrease in system volume, or predetermine a distance to expand based on test specimens and then proceed willy-nilly expanding hundreds of tubes in a nuclear steam generator. Unless, of course, one can accept the degree of control that results.
Similarly, the compressable elastomer device of Rogers, Jr. et al., is, in fact, incapable of controlled expansion of the tube to within 0.006 inches.
The present invention overcomes these disadvantages of the prior art. Fluid pressure is used to expand a distensible polyurethane bladder within the sleeve to expand the sleeve into the tube. The fluid is contained at all times within the bladder, thus there is no spillage and no need for repriming of the expander system.
The degree of expansion of the steam generator tube outer diameter is controlled within 0.006 inches by determining in each case exactly when the steam generator tube begins to yield. This is accomplished by monitoring the change in pressure (dP) of the fluid as a function of the change of the volume (dV) of the fluid system exclusive of the distensible bladder. It is most important to note here that dV represents the volume of the fluid system exclusive of the distensible bladder. According to one embodiment of the invention, the change in pressure, dP, of the system fluid is compared to the change in volume dV. The fluid pressure, P, of the system, will increase linearly relative to dV until the yield point pressure of the sleeve material is reached. As the sleeve yields, the pressure increases at a slower rate relative to dV since the bladder is distending, thus adding volume to the total system and lessening the net decrease in the total volume of the system inclusive of the bladder volume. When the sleeve contacts the inside surface of the steam generator tube, the pressure will increase at a higher rate with respect to dV until the yield strength of the steam generator tube is reached. Again, when the pressure begins to increase at a slower rate with respect to dV the steam generator tube has begun to yield and expand. This is the critical point. The variance of dimensions and material properties of the steam generator tube and the sleeve precludes precise calculation of this point using the prior art methods. By monitoring the pressure rate of change with respect to dV according to the present invention, this point where the steam generator tube begins to yield can be determined with precision in each and every case. In this way, the expansion of the steam generator tube outer diameter can be controlled to within the 0.006 inch tolerance.
In another embodiment, rather than incrementally decreasing the volume of the fluid system, a mass pump adds an incremental fluid mass, dM, to a fluid system having a fixed fluid volume (fixed volume exclusive of the expansion area, as discussed above). Hereinafter a "volume pump" is defined as a positive displacement pump which acts to increase or decrease pressure of a fluid system by controllably effecting the volume of the fluid system. Hereinafter a "mass pump" is defined as a positive displacement pump which acts to increase or decrease the pressure of a fluid system by controllably effecting the mass of the fluid system. Whether a volume pump or a mass pump is used, the fluid system pressure is monitored as a function of the pump incremental action to determine the onset of plastic deformation of the sleeve and tube.
It is an object of the present invention to provide a method of controllably expanding tubes within a 0.006 inch limit.
It is a further object of the present invention to provide a method having the foregoing advantage and which determines the yield point of the tube on a case-by-case basis.
It is a further object of the present invention to provide a hydraulic tube expander utilizing an expandable bladder to thereby contain the hydraulic fluid to prevent fluid spillage and eliminate the need to clean the steam generator or to reprime the apparatus between expansions.
Other objects and advantages of the present invention will be readily apparent from the following description and drawings which illustrate preferred embodiments of the present invention.